1. Field of the Invention
The present invention relates to a new oxetane derivative. It relates more particularly to a polymerizable oxetane derivative, which is an optically active compound that can be used as a chiral agent, and also relates to a liquid crystal composition including the oxetane derivative, a polymer obtained from the composition, and their use.
2. Related Art
Cholesteric liquid crystal molecules have a helical structure in their liquid crystal state. Because of this, they reflect circularly polarized light with a specific wavelength range, depending on the rotating direction and the pitch length of the helix of the liquid crystal molecules, when the helical structure fixed by the polymerization of the cholesteric liquid crystal molecules is irradiated with light. For example, light with wavelengths of blue, green, yellow and red, which correspond to the pitch length of liquid crystals, is selectively reflected on irradiation with visible light. These color tones are different from those of pigment or dye, in which colors arise from light absorption, and have viewing angle-dependence in which a color tone varies on the basis of viewing angles. Moreover, since the pitch length of the cholesteric liquid crystals can be adjusted according to the temperature and the kind of the liquid crystals, not only visible light but also light in the infrared and ultraviolet ranges can be selectively reflected.
Materials that selectively reflect light with various wavelengths in a wide wavelength range have been used by utilizing such characteristics of the cholesteric liquid crystals. The materials are included in liquid crystal pigments, coating materials, spray inks, printing inks, cosmetics, anti-counterfeit printed matters, ornaments, and the like. Use of the materials for an optical film such as a polarizing plate, a compensation plate and a color filter in an optical device such as a liquid crystal display device and a holographic device has been proposed. With respect to the existing liquid crystal pigment, a flaked cholesteric liquid crystal polymer and microcapsulated cholesteric liquid crystals have been used. These usages are for automobile coatings, cosmetics and so forth.
Cholesteric liquid crystals can usually be prepared by the addition of an optically active compound (a chiral agent) to nematic liquid crystals. The cholesteric liquid crystals require a helical structure with a quite short helical pitch in order to reflect circularly polarized light in the range of ultraviolet light to visible light. In this case, the optically active compound is required to have a large helical twist power (a HTP). When an optically active compound with a small HTP is used, the amount of the compound should be increased. However, an increase of the amount causes disappearance of liquid crystallinity of the mixture with cholesteric liquid crystals and an objective cholesteric phase cannot be obtained, since an ordinary optically active compound has no liquid crystallinity. Thus, it is difficult to adjust characteristics, especially the temperature range of a cholesteric phase or a wavelength range that allow selective reflection, when the HTP of the optically active compound is small.
When an optical film is produced, a polymerizable chiral agent is added to a polymerizable liquid crystal compound, and a polymer is formed, fixing a helical structure. Such a chiral agent is required to have characteristics such as a large HTP, a small temperature dependence of the HTP, an excellent compatibility with the liquid crystal compound and an excellent long-term stability (to be hardly crystallized). Even in the case where the chiral agent has a large HTP, it may be unsuitable for industrial use when it has a complicated structure, since the preparation is not easy and thus the cost is raised. The polymer is required to have an excellent balance as requested in characteristics such as mechanical strength, coating property, solubility, degree of crystallinity, shrinkage, water permeability, gas permeability, melting point, glass transition temperature, thermal resistance and chemical resistance.
Polymerizable oxetane derivatives are disclosed in JP H08-020641 A (1996), (patent document 1), JP 2009-120522 A (patent document 2) and Journal of Materials Chemistry, 2002, 12, 2225-2230 (non-patent document 1). Now, the development of a new oxetane derivative is desired.